The present invention relates to a starter device for internal combustion engines, including a movable element slidable in a hollow support structure and carrying a movable electrical contact for cooperating with fixed contacts carried by the structure, during starting.
Starter devices of the aforesaid type are used in electromagnetically-operated electric starter motors, the movable element being constituted by the core of an electromagnet which operates the lever for engaging the starter motor, or by a rod located in front of the core of the electromagnet on the axis thereof and adapted to be operated by the core when it is near one of its end-of-travel positions.
In the first case (a movable electrical contact fixed to the core of the electromagnet) it is necessary to make the movable electrical contact perform the same operating stroke as the core of the electromagnet. This involves problems with the bulk of the electrical switch associated with the electromagnet for operating the starter motor.
In the second case (a movable rod separate from the core of the electromagnet for operating the electrical contact) a more compact switch can be provided since the core of the electromagnet comes into contact with an end of the rod only near the end of its travel in order to close the contact. Whilst it has been found to be very reliable and compact, this solution may involve the risk of undesirable "sticking" of the movable contact to the fixed contacts since it lacks the considerable pull of the electromagnet which is returned to its rest position after starting by means of a suitable spring. The separation of the contacts is therefore entrusted to a weak spring that urges the rod on which the movable electrical contact is mounted towards a rest position in which it is spaced from the fixed electrical contacts. The risk of "sticking" of the contacts results from the considerable acceleration stresses to which the electrical contacts are subjected when they are closed. These impact stresses cause the movable contact to "bounce" repeatedly on the fixed contacts, causing successive openings/closures of the electrical circuit and high voltage peaks resulting in electrical arcs which may lead to localised fusion of the metal and consequent welding of the contacts.